1. Field of the Invention
The present invention relates to a surface acoustic wave filter including resonators, a duplexer, and a multiplexer.
2. Description of the Related Art
Hitherto, there has been developed a branching device in which a reception frequency band is located on a higher side relative to a transmission frequency band (see, for example, International Publication No. 2013/046892).
The branching device described in International Publication No. 2013/046892 is a duplexer including a ladder transmission filter having a plurality of series arm resonators and a plurality of parallel arm resonators. In the transmission filter, the parallel arm resonators include a first resonator having a resonant frequency which is lower than the resonant frequency of the series arm resonators, and a second resonator having a resonant frequency which is higher than the resonant frequency of the series arm resonators and an electrostatic capacity which is lower than the electrostatic capacities of the series arm resonators and the other parallel arm resonators. By setting the resonant frequency of the second resonator to be higher than the resonant frequency of the ladder transmission filter, isolation characteristics within a wide-side bandpass area of the transmission filter are improved.
In a related-art branching device, a high-frequency-suppression trap is provided for a frequency at which the signal level is desired to be decreased, and attenuation characteristics are made appropriate and a desired signal level is secured within the band of the branching device. In this branching device, however, there may arise so-called rebounding of the signal level, in which the attenuation characteristics become deteriorated at a frequency outside the band which is different from the frequency to be suppressed by the trap. Thus, a problem arises in that the filter characteristics become deteriorated.